The invention applies in particular to images in accordance with the JPEG2000 standard.
According to this standard, a compressed digital image signal has a general structure comprising a main header and a body which comprises, in compressed form, original data representing physical quantities which are the pixels and which are organized in blocks of data (known as “codeblocks”) ordered in the signal.
Each compressed data block is a compressed representation of an elementary rectangular part of the image signal which has, for example, been transformed, in a known manner, into frequency sub-bands.
In general terms, a compressed digital image signal according to the JPEG2000 standard thus contains several sets of compressed data blocks each corresponding to a given tile (if the image signal is decomposed into a tile or tiles), component (e.g. luminance or chrominance), resolution level, quality level and spatial position.
In addition, it is known to apply geometric transformations to compressed images for various reasons, these images not necessarily being in accordance with the JPEG2000 standard.
In general terms, the compression of an image involves a necessary coding step, for example of the entropy encoding type.
The coding step is often preceded by a spatio-frequency transformation step (for example of the Discrete Cosine Transform or DCT type) and a step of quantizing the coefficients issuing from the transformation.
In order to apply a geometric transformation to a compressed image signal, generally a complete decompression of the compressed image signal is carried out by applying to the latter the operations which are the reverse of those described above: entropy decoding, dequantization and applying a transformation which is the reverse of the spatio-frequency transformation.
Next the geometric transformation, such as for example a reflection in a vertical axis, is applied to the decompressed image signal, that is to say in the image domain.
Provision is then made for once again compressing the transformed image signal, also referred to as the transcoded signal, by applying successively the operations of spatio-frequency transformation, quantization and entropy coding.
However, the method which has just been described does not take account of the memory space necessary for performing all the above mentioned operations solely for the purpose of applying a geometric transformation to the compressed image signal.
However, some data processing apparatus such as, for example, digital cameras, camcorders or personal digital assistants (PDAs), do not have sufficient memory space for performing all these operations using a compressed image signal.
The memory space necessary may also vary according to the image signal in question and, in particular, its size, the number of its components, the number of bits allocated to each component etc.
Having regard to the above, it is therefore sometimes impossible to perform geometric transformations using compressed image signals in data processing apparatus whose memory capacity proves to be insufficient.
It would consequently be advantageous to be able to perform a geometric transformation on an image signal using compressed image signal, and this in a data processing apparatus whose memory capacity is relatively small.
Further, in some image compression standards, it is not always possible to perform all the required geometric transformations.
Thus, for example, Part 1 of the JPEG2000 standard provides for the possibility of performing only transpositions on the compressed images, whilst Part 2 of this same standard for its part makes provision for being able to perform seven types of different geometric manipulations on the images.
Because of this, when a data processing apparatus (digital camera, camcorder etc) receives compressed images in accordance with Part 1 of JPEG2000 and compressed images in accordance with Part 2 of this standard, and if it is wished to pass from a geometric representation of the image in portrait mode to a representation in landscape mode, or vice-versa, it is necessary to adapt the processing of the compressed image to the part of the standard in question, which complicates the functioning of the apparatus.
This problem of adaptation of the processing of the compressed image may also be encountered with other compression standards.
A complication in the functioning of the apparatus proves prejudicial to apparatus whose processing capacity is relatively small.